Effects of poling camber angle on the biomechanics of cross-country sit-skiing

Cross-country sit-skiers use double poling (DP) technique to drive the slide. The aim of this study is to analyze how poling camber angle affect the capacity of power output and biomechanical parameters of the DP process. Twenty-four non-disabled college students (24.67 ± 1.46 years old) were recruited to perform three successive 30-s maximal effort tests with different poling camber angles of 0°, 15°, 24° and 30° using a sit-skiing ergometer. The biomechanical parameters, output power and muscle activation of the subjects were analyzed. The results showed that DP output power increased with the increase of poling camber angle at 15° (597.78 ± 150.31 J), 24° (610.94 ± 158.96 J, P = 0.011) and 30° (629.10 ± 168.78 J, P < 0.001) compared with 0° (590.65 ± 148.95 J). However, effective output power decreased with the increase of camber angle. Poling with camber angle of 24° had the shortest cycle time 1.53 ± 0.17 s, compared with other abduction angle (0°, 1.57 ± 0.19 s, 15°, 1.55 ± 0.16 s, and 30°, 1.56 ± 0.19 s). Compared with 0° (1.02 ± 0.14 m), the cycle distance significantly increased at poling camber angles of 24° (1.07 ± 0.12 m, P = 0.029) and 30° (1.11 ± 0.13 m, P < 0.001). With the increase of poling camber angle, the shoulder and elbow joint range of motions and joint moments were significantly increased. This study found that poling with shoulder abducted increased the output power but decreased the efficiency. By analyzing the poling angle and poling force, we find that the optimal poling camber angle may depend on the terrain or the skiing speed. These results may guide the competition techniques and tactics in the matches, and may further influence the strength-training programs of cross-country sit-skiing athletes.

The above studies in dissecting the poling technique of cross-country sit-skiing meanly focused on the technique in the sagittal plane.During these tests, athletes performed the DP in a standard technique, with the shoulder flexion and then extension to drive the sled.But, it was not always the case in the races, that athletes were observed to pole with camber angle in the Paralympic Games of cross-country sit-skiing 19 .During a competition, athletes attempt to glide with the pole inclined out.With shoulder abducted during the poling, the pole force will be resolved to a vertical, a propulsive and a mediolateral component.The propulsive component is function of the poling camber angle: which means that the more the shoulder abducts with the greater poling camber angle, the greater the loss of the propulsive force.This is an import factor that determining the efficiency of DP in cross-country sit-skiing, but we hardly found any relevant studies on this issue.
There are similar problems in the wheelchair propulsion process, as we found that the wheels of sports wheelchair are not installed vertically, but have an inclined angle with the ground [20][21][22] .Mason et al. 23 found that a wheelchair camber angle of 24° produced greater power than camber angles of 15° and 20°.While another study found that a camber angle of 18° shows a good effect on the maximal effort mobility performance in wheelchair athletes 24 .These studies showed that poling camber angle can affect the propulsion process of wheelchair user.
The purpose of this study is to investigate the effect of poling camber angle on biomechanical parameters during DP propulsion of cross-country sit-skiing.By using a self-designed ergometer, athletes were asked to perform DP with poling camber angle, and evaluate how poling camber angle affect the capacity of power output and biomechanical parameters of cross-country sit-skiing.The results of the study can help to understand the effects of poling camber angle on propulsion mechanics, and may provide instructions on the training of cross-country sit-skiers.

Participants
Twenty-four non-disabled students were recruited from the Capital University of Physical Education and Sports, who are 12 females and 12 males, 24.67 ± 1.46 years old, 168.75 ± 6.78 cm height and 62 ± 10.39 kg in mass.Inclusion criteria were as follows: participants who majored in physical training.The subjects suffering from injuries during the testing period were excluded.
This study was approved by the Ethics Committee of the Capital University of Physical Education and Sports (Beijing, Peoples' Republic of China), and all experiments were performed in accordance with relevant guidelines and regulations.Before the start of the study, participants were informed about the design of the study, with a special emphasis on the possible risks and benefits, and all participants provided written informed consent prior to the enrolment.

Physical test
All tests were performed on the intelligent training and experimenting system of cross-country sit-skiing (Fig. 1A) in the lab environment with a temperature of 23 °C and suitable humidity.Before the formal test, subjects were introduced to the testing equipment and allowed to warm-up at self-selected resistance and poling rhythm for 5 to 10 min.After the warm-up, reflective marker points and surface EMG sensor electrodes were fixed by using strips of adhesive Velcro, and maximum voluntary contraction (MVC) exercises were performed for each muscle to normalize EMG signals.
Subjects were asked to sit with their lower leg strapped to the seat to simulate the conditions of athletes with disabled lower limb, and then performed three successive 30-s maximal effort tests (poling camber angle at 0°, 15°, 24°, 30°) of DP with 3 min rest between each group and the three consecutive tests.The poling resistance was set at 5% of body weight, which was chosen from several pilot testing and gave skiers the most natural feeling of double-poling 8 .

Equipments and data collection
Physical tests were performed on an intelligent training and experimenting system of cross-country sit-skiing (Fig. 1) 8 .The device can adjust the angle of two rails, which offers non-parallel rails to simulate poling with different camber angles.By using the new system, participants performed DP to simulate the cross-country sit-skiing propulsion process with different poling gestures.The poling forces were recorded at 50 Hz.

Data analysis
During the cross-country sit-skiing simulation, a DP cycle is divided into a poling phase (PP) and a return phase (RP).PP was defined with start with the pole tips in its foremost position and ending in their rearmost position; the RP was defined as the opposite.Cycle time (CT), poling time (PT), and recovery time (RT) were the duration of the DP cycle, PP, and RP respectively.All biomechanical variables were calculated from six consecutive stable DP cycles in the middle stage of the poling simulation.The data were collected from the right side of the subject.
Joint angles were defined, in the anatomical position as follows: shoulder = 0° (flexion positive, extension negative), elbow = 0° (flexion positive), spine flexion = 0° (angle between pelvis and trunk in sagittal plane, kyphosis/ flexion negative), and pole angle in sagittal plane where horizontal (= 0°) and pole angle vertical (= 90°) 16 .We affixed marker points to the slider and the mean distance of a PP was measured by monitoring the markers sticking on the poling platform using the Qualisys system.The cycle distance is the total distance between the PP and the RP.Output power during the poling cycle was defined as: Output Power = barbell movement distance × barbell gravity, and the barbell movement distance was represented by the marker movement sticking on the poling platform.Effective power was defined as By using OpenSim software, individual size and body proportions were used to scale the model.Scaling is typically performed by comparing experimental marker data to virtual markers placed on a model.In addition, the segment masses and inertia properties were scaled from body mass 26 , so that they better match the experimental data.A digital low-pass filter with a cut-off frequency of 13 Hz was used to filter the measured kinematics data.Then the inverse kinematics (IK) was performed to estimate the joint angles of the particular subject from experimental data.Poling reaction force was simulated by a point force applied to the hand.Then, inverse-dynamics equations were solved using kinematic data and external force data.Finally, static optimization (SO) was used to calculate the muscle forces.
EMG signals were integrated by a bandpass filter ranging from 50 to 300 Hz and then processed with fullwave rectification and linearly enveloped, and finally normalized by MVC 16 .For each cycle the average muscle activity (aEMG) and the muscle activity peak (EMGpeak) were calculated on the rectified signals and on the linear envelope respectively 3 .

Musculoskeletal modeling
We used a bilateral upper extremity trunk model for cross-country sit-skiing double poling propulsion 27 .The cross-country sit-skiing model is developed by combining three previously built OpenSim models.In brief, fullbody lumbar spine (FBLS) 28 was adopted as the base model; the DAS3 model 29

provides rotator cuff muscles
Effective power = output power cos(camber angle) .

Statistical analysis
Values were recorded as mean values ± standard deviations (SD).All data are averaged from three tests.Data were checked for normality by using the Shapiro-Wilk analysis.The data were analyzed by one-way ANOVA of repeated measures, with Bonferroni post-test to determine the poling camber angle at which the difference across trials occurred.Statistical significance level was set at P < 0.05.All data were analyzed by using SPSS version 26.0.

Temporal-spatial parameters
The results of the temporal parameters of ski propulsion are listed in Table 1.Athlete poling with camber angle of 24° had the smallest CT (t = 1.53 ± 0.17 s) and PT (t = 0.73 ± 0.07 s).The results revealed that the abduction angle affects the cycle distance, in detail, the cycle distance with poling camber angle of 24° (1.07 ± 0.12 m, P = 0.029) and 30° (1.11 ± 0.13 m, P < 0.001) were significantly larger than 0° (1.02 ± 0.14 m) (Fig. 2A).

Discussion
To the best of our knowledge, this is the first study to analyze the effect of poling camber angle on the biomechanics of cross-country sit-skiing.We found that the output power increased when poling with the camber angle, as the temporal-spatial parameters such as CT and PT were decreased and cycle distance were increased.Joint flexion of ROM and joint moment of shoulder and elbow were also influenced by the poling camber angle.In addition, Antdelt and Bic were found to be sensitive to the poling camber angle.These findings add to an emerging body of literature dissecting how DP technical skills contributes to the cross-country sit-ski.

ROM (°) 0°15°24°30°S
houlder The temporal-spatial parameters show that when athletes pole with camber angle, they pole faster (smaller PT and CT) and longer (longer cycle distance).The increase of cycle distance may attribute to the increase of elbow ROM, and the ROM of elbow and spine joint was increased with the poling camber angle.A previous study produced by Ohlsson et al. 16 underscores that power generation predominantly occurs in the trunk, implying a potential amplification in trunk power.As when athlete pole with camber angle, the elbow get more flexed and extended at the start and end of the poling phase respectively (Fig. 3B).During the PP the shoulder joint exhibits a flexion-extension pattern similar with the elbow 31 .To flex more at the start of the poling phase is easy, but the upper limb did not output power at RP.During the RP, the athlete tries to lift the pole until they support it again.To extend more at the end of the poling phase is hard, as shoulder and elbow have to export larger extension moment (Fig. 3D,E), this is because the moment arm (both shoulder and elbow joint) increased as the elbow extended.In addition, as the upper limb swing more backward, the 3-dimensional pole force has a greater component along the forward direction, which makes the poling more efficient 12 .This is consistent with the output power results in Fig. 2B.
The output power increased when poling with shoulder abducted, and naturally the muscle activation level and muscle forces increased, especially for the deltoids and the Bic (Fig. 4).The deltoids are the stabilizing muscles of shoulder joint 32 , and generate the largest forces during the PP (Fig. 4).As the functional muscle of shoulder abduction, the maximum muscle forces of Antdelt significantly increased when poling with shoulder abduction.In addition, the increase of Bic muscle force may be due to the increase of elbow flexion during the RP, as the RT decreased when poling with the poling camber angle.
Although the results showed that the larger the poling camber angle, the larger the of output power, the poling camber angle not be too large.As we find that at the later stage of the poling cycle, there is a positive correlation in the late stage of DP cycle, the shoulder abduciton angle is about 33° when the camber angel are larger than 30° (Fig. S3).When the shoulder abduction angle is larger than 30°, the stability of the shoulder joint will be reduced and leads to a significant risk of shoulder injury 33 .Meanwhile, the effective output power decreased with a larger camber angel, and a larger shoulder abduction angle at the later stage of the poling cycle (Fig. 2).A larger camber angle decreased the pole angle, particularly at the start of the PP (Fig. S1), which means that the vertical component of pole force is increased.This can be beneficial to the uphill terrain that larger vertical component of the pole force will counteract greater gravity force.
In should be noted that this study has a few limitations.Professional athletes with lower limb impairments were not recruited for this research.For non-disabled athletes, securing the lower legs to the seat may restrict knee movement, potentially impacting the engagement of leg muscles and influencing the flexion angle of the spine.The simulation in the laboratory can not completely simulate the real competition in the snow, in addition, the researchers also did not consider longer testing of DP on the ergometer, which could make the testing more similar to the real situation of a cross-country sit-skiing race.

Conclusion
The current study revealed that it is beneficial to increase cycle distance and poling speed as the angle of abduction increases.And poling with camber angle increased the output power but decreased the efficiency.These results means the optimal poling camber angle may depend on the terrain or the skiing speed.In detail, crosscountry sit-skiers would be recommended to pole with a larger poling camber angle at the start up.Poling with shoulder abducted is beneficial for the muscle power generation, and with the larger vertical component of the pole force counteract with the gravity force, it's easier to drive the sled at an uphill terrain.And at the flat terrain,

Figure 1 .
Figure 1.Equipment and the schematic diagram of double poling technique.(A) The intelligent training and experimenting equipment of cross-country sit-skiing.(B) The schematic diagram of double poling with shoulder abducted.(C) Action division in PP and RP.

Table 1 .Figure 2 .
Figure 2. Cycle distance and output power of different poling camber angles.Cycle distance (A), output power (B), effective output power (C) of the double poling cycle in 30-s experiments.

Figure 3 .
Figure 3. Joint angle and joint moment of shoulder, elbow and spine during the double poling (DP) cycle in cross-country sit-skiing.(A-C) Joint angles of shoulder flexion (A), elbow flexion (B), and spine flexion (C).(D-F) Joint moment of shoulder flexion (D), elbow flexion (E), and spine flexion (F).All angles are measured in degrees and are presented for 100% double poling (DP) cycle.